The present invention relates to a process for producing a material for an organic electroluminescent device by implanting or injecting an emission center-forming compound by molecular implantation with laser, a material for an organic electroluminescent device obtained by this process, and to an organic electroluminescent device (elements) produced with this organic electroluminescent device material.
Electroluminescent devices (hereinafter, occasionally referred to simply as EL devices) have generally been classified as inorganic EL devices or organic EL devices according to the materials they are made from. Some inorganic EL devices utilizing inorganic fluorescent molecules are in practical use partly, and have been brought into application to the backlight of clocks or the like. Meanwhile, organic EL devices have been desired to be brought into practical use because of their being more excellent in high brightness or luminance, high efficiency, and high-speed response than inorganic ones.
Electroluminescent devices are constituted of a compound or compounds having an electron-transporting function, a hole-transporting function, and an emission center-forming function. As for their structures, there have been reported devices of the single-layer mode having a single layer provided with all the functions mentioned above, and those of the multilayer-mode composed of layers each having different functions. The principle of light emission is considered to be based on the phenomenon that electrons or holes injected from a pair of electrodes recombine within a light-emitting layer to form excitons, exciting the molecules of a light emissive material for the light-emitting layer.
As a compound constituting each layer, a low-molecular weight compound of high light-emission efficiency, a macromolecular compound having a high physical strength, or the like is employed. When a low-molecular weight compound is used, a film is formed by means of a vapor deposition technique, while a macromolecular compound is formed into a film by coating or applying a solution in many cases.
Japanese Patent Application Laid-Open No. 96959/1996 (JP-A-8-96959) and Japanese Patent Application Laid-Open No. 63770/1997 (JP-A-9-63770) disclose organic EL devices comprising a single light-emitting layer made of a polymer binder having both electron-transporting function and hole-transporting function, within which varieties of fluorescent dyes (or colorants, pigments) are dispersed. These organic EL devises are reported to present, as a whole, white light due to the light emission of each light-emitting compound independent of one another. Moreover, as compared with organic EL devices of the multilayer-mode, those of the single-layer mode are hardly deteriorated in light-emission intensity.
Fine patterning, particularly multicolor patterning (full-coloration) of these organic EL devices is difficult because, in their fabrication, a film is formed by means of a solution coating technique in which a solution of a polymer binder and a fluorescent dye(s) dispersed in a specific solvent is applied onto a substrate.
As multicolor patterning methods, a color filter method, a color-converting method, the ink-jet method by T. R. Hebner (Appl. Phys. Lett. 72, 5 (1998), p.519), the photobleaching method by Kido, et al, and others have been reported.
The color filter method or color-converting method has the advantage of not requiring the patterning of a light-emitting layer, but suffers deterioration in conversion efficiency caused by the use of a filter. In the ink-jet method, since a pattern formed by ink-jet printing shows a center-raised, i.e., conical profile and is inferior in smoothness of its surface, it is difficult to provide electrodes thereon uniformly. Moreover, the cross section of the pattern is desired to be rectangular, but that of a pattern by ink-jet printing is circular. Further, the dimensions of a pattern largely depend on conditions under which the pattern is dried and the concentration of the solution. In the photobleaching method, only a special emission center-forming compound which loses its fluorescence upon UV oxidation is employable and therefore colors expressible by EL devices are limited.
As was described above, in conventional film-forming methods by solution coating, although it is possible to use a macromolecular compound with a high physical strength, it is difficult to provide fine patterns. In addition to that, also in the above-described patterning methods, compounds that can be used are limited, and films having surface smoothness suitable for organic EL devices cannot be obtained.
As the molecular implantation technique, Japanese Patent Application Laid-Open No. 297457/1994 (JP-A-6-297457) discloses a method comprising a step of, with a functional material or a solid material containing a functional material (A) and a solid material into which a functional component is to be implanted (B) placed such as to face each other, irradiating a laser pulse thereby to implant the functional component into the solid material (B). The literature describes that a position to be implanted of the functional component can be controlled by adjusting an irradiation position of the laser.
Japanese Patent Application Laid-Open No. 106006/1996 (JP-A-8-106006) discloses a method comprising the steps of bringing a source film of an organic macromolecular compound within which dyes absorptive of a pulse laser are dispersed into tight contact with a target film of an organic macromolecular compound transmittable of a pulse laser, and irradiating a pulse laser from the target film side with an intensity of or below the ablation threshold value of the source film thereby to implant the dyes into the target film. This literature says that the molecular implantation technique can be utilized in the fabrication of color filters for use in displays or the like. Moreover, the literature describes that an image can be formed by moving a spot position of the laser, or the source film and the target film. In Examples of the literature, moving a sample in parallel forms a linear image.
Japanese Patent Application Laid-Open No. 150158/2000 (JP-A-2000-150158) discloses a process of producing a material for use in an organic electroluminescent device, in which a source containing an emission center-forming compound absorptive of a laser beam is brought into contact with a target having an electron-transporting function and/or a hole-transporting function and the source is irradiated with a pulsed laser beam with an intensity not higher than the ablation threshold of the source thereby to inject the emission center-forming compound into the target. This specification describes that irradiation of a laser beam through a photomask realizes unrestricted setting of a pattern form.
Accordingly, an object of the present invention is to provide a material for organic EL device (particularly, an organic EL device-use film) capable of minute and fine patterning even when a macromolecular compound is used as an EL device-use material and capable of implanting or injecting the emission center-forming compound simply and efficiently, a process for producing the same, and a material for an organic EL device, which is obtained by the process.
Another object of the present invention is to provide a material for an organic EL device, which is excellent in surface smoothness and has good contactness with electrodes, and an organic EL device using the same.
The inventors of the present invention made intensive and extensive studies to achieve the above objects, and finally found that, in a molecular implantation technique using a source constituted of an emission center-forming compound, fine patterning is realized simply and effectively by a process comprising (1) implanting or injecting an emission center-forming compound into a target by relatively moving a laser beam relative to the target, or (2) implanting or injecting an emission center-forming compound into a target with the use of an interference light of a laser. The present invention was accomplished based on the above findings.
That is, a process for producing a material for an organic electroluminescent device of the present invention comprises implanting an emission center-forming compound constituting a source into a target having at least one function selected from the group consisting of an electron-transporting function and a hole-transporting function by irradiating a laser beam, and wherein (1) the laser beam and at least the target are relatively moved with irradiating the beam to form the emission center with a predetermined pattern, or (2) the emission center at an area, corresponding to an interference pattern of an interference light, of the target is formed by irradiating the interference light of the laser beam.
In the method (1), the source and the target being in contact with each other may be moved relative to the laser beam for forming the emission center with a predetermined pattern. Moreover, the irradiation of the laser beam may be conducted through a waveguide, or the irradiation of the laser beam may be conducted with the use of an optical fiber. Further, the laser beam and the target may be relatively moved with irradiating the beam and with moving the source relative to the beam for forming the emission center. The laser beam may comprise a pulse laser beam. The laser beam and the target may be relatively moved with synchronizing the beam with a cycle of the pulse.
In the method (2), a laser beam from a single light source may be split into a plurality of light paths to cause the interference by an optical path difference. For example, the interference may be caused by introducing the laser beam into a hole or a slit, or the interference may be caused by reflecting the laser beam through a plurality of reflection paths.
The methods (1) and (2) may comprise irradiating the laser beam with an intensity of or below the ablation threshold value of the source. The laser beam may comprise a pulse laser beam. The target may comprise an organic polymer. The target may comprise a compound having at least one function selected from the group consisting of the electron-transporting function and the hole-transporting function, and a film-formable organic polymer. The compound may comprise an oxadiazole derivative having the electron-transporting function, and/or an aromatic tertiary amine having the hole-transporting function.
The present invention also includes a material for an organic electroluminescent device, which is obtainable by the above-mentioned process, and an organic electroluminescent device using the material for an organic electroluminescent device.